Thioesters are known to form in a variety of metabolic processes including fatty acid oxidation (Genschel, U. (2004) Mol. Biol. Evol. 21, 1242-1251), protein splicing (Gogarten, J. P., et al. (2002) Annu. Rev. Microbiol. 56, 263-287), and activation of enzyme intermediates (e.g., ligases). Cys thiols within numerous mammalian proteins are also posttranslationally modified by long-chain fatty acid (predominantly palmitate), which is added enzymatically via thioester→thioester transesterification from an activated acylCoA donor, and which is thought to play a role predominantly in subcellular localization (Dietrich, L. E. & Ungermann, C. (2004) EMBO Rep. 5, 1053-1057).
A variety of plant constituents, many of which are abundant in the diet, contain α,β-unsaturated carbonyl groups (Michael acceptors), which have been shown to react with critical cysteines in target proteins such as in the Nrf2/Keap system (Dinkova-Kostova, A. T., et al. (2002) Proc. Natl. Acad. Sci. USA 99, 11908-11913, Wakabayashi, N., et al. (2004) Proc. Natl. Acad. Sci. USA 101, 2040-2045, Talalay, P. & Fahey, J. W. (2001) J. Nutr. 131, S3027-S3033). These interactions result in irreversible alkylation via formation of a thioether bond.
As shown herein, avicins, a family of plant-derived glycosylated pentacyclic terpenoids, contain not only Michael acceptor sites but also reactive oxyesters, which participate in transesterification to yield a protein adduct linked by a reversible thioester bond.